fix: mock Agent querying OS for listening ports in tests (#20842)

fixes https://github.com/coder/internal/issues/1123

We want to tests that ports are not included after they are no longer used, but this isn't safe on the real OS networking stack because there is no way to guarantee a port _won't_ be used. Instead, we introduce an interface and fake implementation for testing.

On order to leave the filtering logic in the test path, this PR also does some refactoring.

Caching logic is left in the real OS querying implementation and a new test case is added for it in this PR.

agent/agent.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"hash/fnv"
 	"io"
+	"maps"
 	"net"
 	"net/http"
 	"net/netip"
@@ -70,16 +71,21 @@ const (
 )
 
 type Options struct {
-	Filesystem                   afero.Fs
-	LogDir                       string
-	TempDir                      string
-	ScriptDataDir                string
-	Client                       Client
-	ReconnectingPTYTimeout       time.Duration
-	EnvironmentVariables         map[string]string
-	Logger                       slog.Logger
-	IgnorePorts                  map[int]string
-	PortCacheDuration            time.Duration
+	Filesystem             afero.Fs
+	LogDir                 string
+	TempDir                string
+	ScriptDataDir          string
+	Client                 Client
+	ReconnectingPTYTimeout time.Duration
+	EnvironmentVariables   map[string]string
+	Logger                 slog.Logger
+	// IgnorePorts tells the api handler which ports to ignore when
+	// listing all listening ports. This is helpful to hide ports that
+	// are used by the agent, that the user does not care about.
+	IgnorePorts map[int]string
+	// ListeningPortsGetter is used to get the list of listening ports. Only
+	// tests should set this. If unset, a default that queries the OS will be used.
+	ListeningPortsGetter         ListeningPortsGetter
 	SSHMaxTimeout                time.Duration
 	TailnetListenPort            uint16
 	Subsystems                   []codersdk.AgentSubsystem
@@ -137,9 +143,7 @@ func New(options Options) Agent {
 	if options.ServiceBannerRefreshInterval == 0 {
 		options.ServiceBannerRefreshInterval = 2 * time.Minute
 	}
-	if options.PortCacheDuration == 0 {
-		options.PortCacheDuration = 1 * time.Second
-	}
+
 	if options.Clock == nil {
 		options.Clock = quartz.NewReal()
 	}
@@ -153,30 +157,38 @@ func New(options Options) Agent {
 		options.Execer = agentexec.DefaultExecer
 	}
 
+	if options.ListeningPortsGetter == nil {
+		options.ListeningPortsGetter = &osListeningPortsGetter{
+			cacheDuration: 1 * time.Second,
+		}
+	}
+
 	hardCtx, hardCancel := context.WithCancel(context.Background())
 	gracefulCtx, gracefulCancel := context.WithCancel(hardCtx)
 	a := &agent{
-		clock:                              options.Clock,
-		tailnetListenPort:                  options.TailnetListenPort,
-		reconnectingPTYTimeout:             options.ReconnectingPTYTimeout,
-		logger:                             options.Logger,
-		gracefulCtx:                        gracefulCtx,
-		gracefulCancel:                     gracefulCancel,
-		hardCtx:                            hardCtx,
-		hardCancel:                         hardCancel,
-		coordDisconnected:                  make(chan struct{}),
-		environmentVariables:               options.EnvironmentVariables,
-		client:                             options.Client,
-		filesystem:                         options.Filesystem,
-		logDir:                             options.LogDir,
-		tempDir:                            options.TempDir,
-		scriptDataDir:                      options.ScriptDataDir,
-		lifecycleUpdate:                    make(chan struct{}, 1),
-		lifecycleReported:                  make(chan codersdk.WorkspaceAgentLifecycle, 1),
-		lifecycleStates:                    []agentsdk.PostLifecycleRequest{{State: codersdk.WorkspaceAgentLifecycleCreated}},
-		reportConnectionsUpdate:            make(chan struct{}, 1),
-		ignorePorts:                        options.IgnorePorts,
-		portCacheDuration:                  options.PortCacheDuration,
+		clock:                   options.Clock,
+		tailnetListenPort:       options.TailnetListenPort,
+		reconnectingPTYTimeout:  options.ReconnectingPTYTimeout,
+		logger:                  options.Logger,
+		gracefulCtx:             gracefulCtx,
+		gracefulCancel:          gracefulCancel,
+		hardCtx:                 hardCtx,
+		hardCancel:              hardCancel,
+		coordDisconnected:       make(chan struct{}),
+		environmentVariables:    options.EnvironmentVariables,
+		client:                  options.Client,
+		filesystem:              options.Filesystem,
+		logDir:                  options.LogDir,
+		tempDir:                 options.TempDir,
+		scriptDataDir:           options.ScriptDataDir,
+		lifecycleUpdate:         make(chan struct{}, 1),
+		lifecycleReported:       make(chan codersdk.WorkspaceAgentLifecycle, 1),
+		lifecycleStates:         []agentsdk.PostLifecycleRequest{{State: codersdk.WorkspaceAgentLifecycleCreated}},
+		reportConnectionsUpdate: make(chan struct{}, 1),
+		listeningPortsHandler: listeningPortsHandler{
+			getter:      options.ListeningPortsGetter,
+			ignorePorts: maps.Clone(options.IgnorePorts),
+		},
 		reportMetadataInterval:             options.ReportMetadataInterval,
 		announcementBannersRefreshInterval: options.ServiceBannerRefreshInterval,
 		sshMaxTimeout:                      options.SSHMaxTimeout,
@@ -202,20 +214,16 @@ func New(options Options) Agent {
 }
 
 type agent struct {
-	clock             quartz.Clock
-	logger            slog.Logger
-	client            Client
-	tailnetListenPort uint16
-	filesystem        afero.Fs
-	logDir            string
-	tempDir           string
-	scriptDataDir     string
-	// ignorePorts tells the api handler which ports to ignore when
-	// listing all listening ports. This is helpful to hide ports that
-	// are used by the agent, that the user does not care about.
-	ignorePorts       map[int]string
-	portCacheDuration time.Duration
-	subsystems        []codersdk.AgentSubsystem
+	clock                 quartz.Clock
+	logger                slog.Logger
+	client                Client
+	tailnetListenPort     uint16
+	filesystem            afero.Fs
+	logDir                string
+	tempDir               string
+	scriptDataDir         string
+	listeningPortsHandler listeningPortsHandler
+	subsystems            []codersdk.AgentSubsystem
 
 	reconnectingPTYTimeout time.Duration
 	reconnectingPTYServer  *reconnectingpty.Server